The process control and measurement industry employs process variable transmitters to remotely monitor process variables associated with substances such as solids, slurries, liquids, vapors, and gasses in chemical, pulp, petroleum, pharmaceutical, food and other food processing plants. Process variables include pressure, temperature, flow, level, turbidity, density, concentration, chemical composition and other properties. A process variable transmitter can provide an output related to the sensed process variable over a process control loop to a control room, such that the process can be monitored and controlled.
The process control loop can be a two-wire, 4-20 mA process control loop. With such a process control loop, the energization levels are low enough that even under fault conditions the loop generally will not contain enough electrical energy to generate a spark. This is particularly advantageous in flammable environments. Process variable transmitters can sometimes operate on such low energy levels that they can receive all electrical power from the 4-20 mA loop. The process control loop can sometimes have digital signals superimposed on the two-wire loop according to a process industry standard protocol such as the HART® digital protocol.
Process pressure transmitters are used in a variety of applications to sense pressure (absolute, gage, or differential) within a process environment. Additionally, a process pressure transmitter can be used to sense differential pressure from two distinct points, such as at varying elevations along a tank and provide an indication of a fluid level within the tank. The measurement of pressures at two different spaced-apart locations in a process installation sometimes requires the use of one or more remote seals. Further, in some applications, the temperature of the process is so high that physically mounting the pressure transmitter near the process would destroy the pressure transmitter. Thus, remote seals are primarily used in applications where either the points at which the pressure is measured are spaced apart, or the temperature of the process itself is too high.
As used herein, a remote seal system consists of a pressure transmitter, at least one remote process seal assembly, a fluidic coupling between the remote process seal assembly and the pressure transmitter, and a fill fluid in the fluidic coupling. During operation, a thin, flexible diaphragm and fill fluid separate the pressure sensitive element of the pressure transmitter from the process fluid. The coupling, typically a capillary connects the remote process seal assembly to the pressure transmitter. When process pressure is applied, the diaphragm within the remote process seal assembly is displaced transferring the measured pressure through the fill system by way of the coupling to the pressure transmitter element. This transferred pressure displaces a sensing diaphragm in the pressure-sensitive element of the pressure transmitter. This displacement is proportional to the process pressure and is converted electronically to an appropriate current, voltage, or digital output signal such as HART® (Highway Addressable Remote Transducer).
In order to ensure that the pressure sensed within the pressure transmitter is an accurate representation of the process pressure, it is extremely important that the fill fluid be incompressible. While this may seem to be a relatively straight forward design criteria, it is known that certain types of oils will, over time, outgas or develop bubbles therein. Additionally, the metals that form the remote process seal assembly and/or the coupling may outgas, over time, to at least some extent. Currently, these issues are addressed by selecting very high quality fill fluids; pre-processing the fill fluid to decrease the extent to which it may outgas over time; and processing the metals that comprise the remote process seal assembly and/or the capillary tube to decrease the degree to which they outgas as well. Employing all of these techniques enhances the long-term stability of remote process seal systems.
Some applications (specifically the ones that expose the remote process seal assembly to high-temperature such as that over 200° C., and/or high-vacuum) can cause instabilities and/or inaccuracies in the remote process seal system. For example, in the above-noted demanding application, a remote process seal system manufactured according to all of the above-identified manufacturing techniques, may still experience gradually increasing inaccuracies or instabilities after operation of a period of six months or more. While this useful lifetime in the face of such a rigorous application has been acceptable, it would be much better to provide a remote process seal system that could withstand such demanding applications for a substantially longer time. Moreover, providing a remote process seal system that could not only withstand such demanding applications for an extended period of time, but also not require the additional processing steps that are currently provided to reduce or minimize the outgassing of the fill fluid and/or metal would represent a significant improvement to remote process seal systems.